On the other end of the spectrum, I can be running at a steady clip for 8-10 miles and my heart rate maxes out at 152! Usually I am in a great aerobic zone (for me) at 130-140. As a long time exerciser and part-time marathon runner, I have spent years trying to figure this mystery out...am I working hard enough at 140 when the other women in my cycling class are at 182?!
There are so many unknown factors that contribute to our bodies, our make-up, our optimum health. What I hear you asking is if you are training YOUR body in a healthy way. Perhaps the fitness zone and target heart rate charts give us a range of normal, but I believe this is like pregnancy and birth - there is normal based on statistical spreads and then, there is your own personal normal.
Here is my suggestion (because it is what I did - so it is only one humble opinion afterall!): Invest in a heart rate monitor for training. My first marathon coach taught me this trick. How it works is twofold. First, it can help you calibrate your resting heart rate and target heart rate zones - based on you specifically. Then, it monitors your daily heart rate zone while you exercise. (A personal monitor with a chest strap and a wrist watch work much better than the hand-held sensors at the gym.)
By calibrating your resting heart rate (taken while lying down, first thing in the morning), you can see how far the spread is between resting and active. Perhaps, since your HR is high when you work out, you also 'idle' high in general. I know that I discovered my resting HR was 46-48 bpm - which is very slow. So, at 152, I have increased my HR over 100 bpm. While low on the charts for maximum HR, 152 is quite high for me based on my resting zone. Discovering this was good information for me.
The additional benefit of training with a HR monitor is that working out becomes a game. You keep track of numbers, of minutes in a zone, you can balance weekly workouts based on the various zones of intensity. You can have one long endurance work out, two shorter workouts at a high intensity and two or three moderate intensity work outs. Knowing your zones also lets you know when to back off and put in a walking interval to bring your HR back down.
I hope this helps a little. I'm an amateur fitness lover, self taught the hard way! Please take my advice with this in mind...I'm no expert!
But I wish you all the best.